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Electrons Damage Satellites

In January 1994, three geostationary satellites
suffered total failures of their momentum wheel control
circuitry. Services were affected for hours, and one
satellite never fully recovered. At the time it was
well known that satellites could suffer
damage when high energy particles were emitted from the
Sun during an explosive Solar Particle Event (SPE).
Protons and electrons travelling at relativistic speeds
(a significant fraction of the speed of light, c, such as 0.3c)
could impact with orbiting satellites causing damage in
a number of different ways. In this case, however,
no SPE had been observed.

It is now believed
that the events above were associated with long lasting
streams of high energy electrons which are now referred to as
high speed solar wind streams. These occur most frequently
around the time of solar minimum and are closely
associated with coronal holes.
A coronal hole is like a window in the magnetic field
structure of the Sun's corona, that allows solar particles
to flow more freely outwards from the Sun towards the Earth.

What appears to be important in predicting whether an
electron flux is dangerous to satellies is not so much
the electron intensity but the total integrated flux.
The USAF uses empirically derived thresholds to issue
warnings to satellite operators. Damaging conditions are
believed to exist when the daily electron flux (number of
high energy electrons > 2 MeV per square centimetre per
steradian per day) meets either of the following
conditions:

greater than 3 x 108 per day for 3 consecutive days

greater than 109 for a single day

Such conditions often occur about 2 days after the onset
of a large geomagnetic storm.

Damage to satellite systems is caused through a
phenomenon called Deep Dielectric Charging. The high
energy electrons penetrate the spacecraft outer surface
and bury themselves in dielectric materials such as
circuit boards and the insulation in coaxial cables. The
buildup gives rise to intense electric fields which when
they exceed the breakdown potential of the material
produce sudden discharges, like a mini-lightning stroke.
It is this discharge that damages the system, burning
components and destroying semiconductors. Designers can
take measures to minimise the effect of such deep
dielectric charging. This generally involves increased complexity
and weight for the satellite however.